Conventionally, the trajectory of an electron beam is generally modulated by an alternating magnetic field generated by a deflecting yoke, a convergence yoke, a velocity modulation coil or the like before the electron beam emitted from a cathode reaches the screen in a cathode-ray tube.
The deflecting yoke generally is provided at a funnel cone portion of a cathode-ray tube. A phosphor screen in the cathode-ray tube is scanned with an electron beam by deflecting trajectories of the electron beam with an alternating magnetic field generated by the deflecting yoke.
The convergence yoke generally is provided outside of a neck of a cathode-ray tube. The raster distortion is corrected by modulating trajectories of an electron beam with an alternating magnetic field generated by the convergence yoke.
The velocity modulation coil generally is provided outside of a neck of a cathode-ray tube and has a function of making a picture image sharp by preventing the runover of a high brightness portion into a low brightness portion on the phosphor screen by modulating the scanning speed of an electron beam with an alternating magnetic field generated by the velocity modulation coil.
An electrode of an electron gun is positioned between an electron beam and a coil for modulating such electron beam trajectories in a magnetic field at high frequency. Generally, a metallic material having high conductivity such as stainless steel or the like has been used as an electrode material for the electron gun for the purpose of forming an electron lens by applying voltage. The sheet resistivity is, for example, about 2m.OMEGA./.quadrature. in stainless steel SUS304 having a thickness of 0.4 mm.
FIG. 6 shows a structural example of an electron gun portion in a projection monochrome cathode-ray tube as a conventional cathode-ray tube. An anodic electrode 1 is made of stainless steel. In this example, the center of a magnetic field of a convergence yoke 8 is positioned 7 mm apart from the end of a phosphor screen side of the anodic electrode 1. Most of alternating magnetic fields 9 generated by the convergence yoke 8 pass through the anodic electrode 1. A deflecting yoke 16 is provided at a funnel cone portion of the cathode-ray tube. A part of alternating magnetic fields 17 generated by the deflecting yoke 16 passes through the anodic electrode 1 and a cylinder 15 shielding a getter. A velocity modulation coil 18 is arranged in the middle of a pre-anodic electrode 3 and a focusing electrode 2. Most of alternating magnetic fields 19 generated by the velocity modulation coil 18 pass through the pre-anodic electrode 3 and the focusing electrode 2.
When the alternating magnetic fields are generated through such metallic electrodes, an eddy current is generated at the parts of the metallic electrodes. The eddy current loss becomes greater as the frequency of the alternating magnetic fields becomes higher. Consequently, the modulation effect on the electron beam trajectories by the magnetic fields decreases in the high-frequency modulation area.
In the conventional example shown in FIG. 6, for example, a modulation effect on electron beam trajectories by the convergence yoke 8 decreases, since an eddy current is generated at the anodic electrode 1 by the alternating magnetic fields 9 generated by the convergence yoke 8.
In some cases, the electrode is heated by this eddy current loss, thus damaging the neck of the tube. In the case of designing a cathode-ray tube so that the distance between a source of alternating magnetic fields and a metallic electrode of an electron gun is made great in order to prevent such a loss in alternating magnetic fields and heat generation at an electrode, the distance between an electron-beam focusing lens and a phosphor screen becomes inevitably greater and the magnifying power of an electron lens becomes therefore greater. Consequently, there is a problem of decreasing resolution. Particularly, the loss in such alternating magnetic fields becomes greater in a picture display unit having a high deflecting frequency and a wide signal zone such as a high definition television or the like, resulting in hindrance in practical use.